Low NOx combustion

ABSTRACT

Combustion of hydrocarbon liquids and solids is achieved with less formation of NOx by feeding a small amount of oxygen into the fuel stream.

This application claims priority from U.S. Provisional Application Ser.No. 60/380,818 filed May 15, 2002.

This invention was made with United States Government support underCooperative Agreement No. DE-FC26-00NT40756 awarded by the Department ofEnergy. The United States Government has certain rights in theinvention.

FIELD OF THE INVENTION

The present invention relates to combustion of hydrocarbon fuelscontaining bound nitrogen, particularly of coal.

BACKGROUND OF THE INVENTION

Environmental awareness is growing in the U.S. and around the worldleading to increasing public and regulatory pressures to reducepollutant emissions from boilers, incinerators, and furnaces. Onepollutant of particular concern is NOx (by which is meant oxides ofnitrogen such as but not limited to NO, NO₂, NO₃, N₂O, N₂O₃, N₂O₄, N₃O₄,and mixtures thereof), which has been implicated in acid rain, groundlevel ozone, and) particulate formation.

A number of technologies are available to reduce NOx emissions. Thesetechnologies can be divided into two major classes, primary andsecondary. Primary technologies minimize or prevent NOx formation in thecombustion zone by controlling the combustion process. Secondarytechnologies use chemicals to reduce NOx formed in the combustion zoneto molecular nitrogen. The current invention is a primary controltechnology.

In primary control technologies, different combustion strategies areused to control so called “thermal NOx” and “fuel NOx”. Thermal NOx isformed by oxidation of nitrogen molecules, N₂, primarily in combustionair at high temperature. It is the main source of NOx emissions fromnatural gas and light oils that do not contain chemically bound nitrogenspecies. The main control strategy to reduce thermal NOx is to reducepeak flame temperature. Fuel NOx is formed by the oxidation ofnitrogenous species contained in fuel and is the main source of NOxemissions from combustion of coal and heavy oil. The current inventionrelates to improved combustion methods to control fuel NOx emission.

The primary control technology for fuel NOx is commonly called stagedcombustion in which mixing between the combustion air and fuel iscarefully controlled to minimize NOx formation. The formation of NOxfrom fuel nitrogen is based on a competition between the formation ofNOx and the formation of N₂ from the nitrogenous species in the fuelvolatiles and char nitrogen. Oxygen rich conditions drive thecompetition towards NOx formation. Fuel rich conditions drive thereactions to form N₂. Staged combustion takes advantage of thisphenomenon by carefully controlling the mixing of air and fuel to form afuel rich region to prevent NOx formation. To reduce NOx emissions, thefuel rich region must be hot enough to drive the NOx reduction kinetics.However, sufficient heat has to be transferred from the fuel rich firststage to the furnace heat load in order to prevent thermal NOx formationin the second stage.

A conventional low NOx burner (LNB) includes a fuel rich first zone,near the feed orifice, which is mainly controlled by mixing andcombustion of fuel and primary air, and to some extent, additionalsecondary or tertiary air mixed in this zone. For combustion ofpulverized coal the primary air is used to transport the coal particles.

In a second zone, the remainder of the secondary air and any tertiaryair mix with the unburned fuel and products of partial combustion fromthe first stage and complete the combustion. An important processrequirement for staged combustion is to transfer a sufficient amount ofheat from the fuel rich first stage to the furnace heat load to cooldown the combustion products from the first stage. Lower second stagetemperature helps to reduce the conversion of remaining nitrogenouscompounds to NOx and also to prevent thermal NOx formation in the secondstage.

In an aerodynamically staged LNB, all of the combustion air isintroduced from the same burner port or adjacent to the burner port. Themost common configuration of a low NOx coal burner is to have a seriesof annular passages for coal/primary air, secondary air and tertiaryair. The central passage is often used for oil gun or for natural gasfor start up heating. Secondary and tertiary air flows are equipped withswirl generators to impart swirling flows to create a recirculation zonefor flame stability. Air velocities and swirl are adjusted to create arelatively large fuel rich first zone along the axis of the burner,followed by relatively gradual mixing of secondary and tertiary airalong the length of the furnace. Since sufficient air velocities must beprovided to mix fuel and air within the furnace space to completecombustion, it is difficult to create a very large fuel rich zone toprovide a long enough residence time for maximum NOx reduction.

Although the LNB is a fairly inexpensive way to reduce NOx and manyadvancements have been made in the burner design, currently availableversions are not yet capable to reach the emissions limits in pendingregulations of 0.15 lb (as NO₂) per MMBtu of coal fired for utilityboilers.

Those skilled in the art have overcome the limitations of anaerodynamically staged LNB by a globally staged combustion arrangementusing “over fire air” (OFA). OFA is injected separately from a burner ora group of burners to provide a large fuel rich primary combustion zone(PCZ) and a burnout zone (BOZ) where combustion is completed by mixingOFA and unburned fuel and the products of partial combustion from thePCZ. Typically the OFA ports are separated at least one burner portdiameter from the closest burner and several burner port diameters fromthe furthermost burner. Although the fuel and air mixing and the localstoichiometric conditions near the burner port of an individual burnerare similar to those without OFA, a large fuel rich PCZ is formedoutside the combustion air mixing zone near the burner. Due to thephysical separation of the OFA injection ports, the residence time inthe fuel rich PCZ is much longer than that typically obtained in thefuel rich first zone of an aerodynamically staged burner. Thecombination of LNB's and OFA ports has enabled further reduction in NOxemissions.

Low NOx burners and over fire air represent a fairly mature technologyand as such are discussed widely throughout the patent and archivalliterature. Many ideas have been proposed to enhance the effectivenessof LNB's and OFA while minimizing detrimental impacts such as poor flamestability and increased carbon in the ash. Of these ideas two areparticularly relevant: preheating the air to the first stage, andconverting the combustor to oxy-fuel firing.

Both air preheat and oxy-fuel combustion enhance the effectiveness ofstaged combustion for fuel NOx reduction by increasing the temperaturein the primary combustion zone without increasing the stoichiometricratio. Oxy-fuel combustion offers the additional advantage of longerresidence times in the fuel rich region, due to lower gas flows, whichhas been shown to reduce NOx emissions. As discussed above, stagedcombustion uses a fuel rich stage to promote the formation of N₂ ratherthan NOx. Since the reactions to form N₂ are kinetically controlled,both the temperature and the hydrocarbon radical concentration arecritical to reducing NOx formation. For example, if the temperature ishigh and the radical concentration is low, such as under unstaged ormildly staged conditions, NOx formation is increased. When the radicalconcentration is high but the temperature is low, such as under deeplystaged conditions, the conversion of intermediate species such as HCN toN₂ is retarded. When air is added to complete burnout, the intermediatesoxidize to form NOx, therefore the net NOx formation is increased.

Sarofim et al. “Strategies for Controlling Nitrogen Oxide EmissionsDuring Combustion of Nitrogen bearing fuels”, 69^(th) Annual Meeting ofthe AIChE, Chicago, Ill., November 1976, and others have suggested thatthe first stage kinetics can be enhanced by preheating the combustionair to fairly high temperatures. Alternately Kobayashi et al. (“NOxEmission Characteristics of Industrial Burners and Control Methods UnderOxygen-Enriched Combustion Conditions”, International Flame ResearchFoundation 9^(th) Members' Conference, Noordwijkerhout, May 1989),suggested that using oxygen in place of air for combustion would alsoincrease the kinetics. Oxy-fuel combustion, when flame temperature iscontrolled by burner design, further reduces thermal NOx formation bysubstantially eliminating N₂ in combustion air. In both cases the netresult is that the gas temperature in the first stage is increased,resulting in reduced NOx formation. Further, using both air preheat andoxy-fuel firing allows the first stage to be more deeply staged withoutdegrading the flame stability. This allows even further reductions inNOx formation.

Oxy-fuel firing offers a further advantage for LNB's. Timothy et al(“Characteristics of Single Particle Coal Combustion”, 19^(th) Symposium(international) on Combustion, The Combustion Institute, 1983) showedthat devolatilization times are significantly reduced, and the volatileyield is increased, when coal is burned in oxygen enriched conditions.These tests were single particle combustion tests performed under highlyfuel lean conditions, which does not provide information on how muchoxygen is needed to accomplish this under more realistic combustionconditions. The higher volatile yield means that the combustibles in thegas phase increase as compared to the baseline—leading to a more fuelrich gas phase which inhibits NOx formation from the volatile nitrogenspecies. In addition, the fuel volatiles ignite rapidly and anchor theflame to the burner, which has been shown to lower NOx formation. Theenhanced volatile yield also leads to shorter burnout times since lesschar is remaining.

O. Marin, et.al., discuss the benefits of oxygen for coal combustion ina paper entitled “Oxygen Enrichment in Boiler” (2001 AFRC/JFRC/IEA JointInternational Combustion Symposium, Kaui, Hi., Sep. 9–13, 2001). Theyproposed injection of oxygen in the over fire air (also described as“tertiary air” in this paper), to reduce unburned carbon in ash, or Losson Ignition (LOI), without increasing NOx emission. The computersimulation results reported by Marin, et al. compared the baseline aircase and an oxygen enriched case with a high velocity, oxygen enrichedstream in the tertiary air (also termed over-fire air). According toMarin, et.al., “An increase of 5% on heat transfer in the combustionchamber, combined with a 7% absolute increase in char burnout arenoted.” (page 8)

U.S. Pat. No. 4,495,874 discloses oxygen enrichment of primary and/orsecondary air in pulverized coal fired burners in order to increase thesteam rate of a boiler firing high ash pulverized coal. Example 4, indisclosing the effects of oxygen enrichment on NO emissions when burninghigh ash coal, says that oxygen added to the primary air or equally toprimary or secondary air initially increased NO content at about 2%enrichment (which is defined there as 23% O₂ concentration of totalair), but sharply decreased the amount of NO in the flue gas at thehigher enrichments. For example, at 4 percent enrichment, NO wasdecreased by about 18–21 percent. However, there was no resulting NOdecrease when oxygen was added only to the secondary air. In fact, therewas an increase in NO concentration of about 12 percent.

Although the prior art describes several elegant enhancements for stagedcombustion and LNB's, several practical problems have limited theirapplication. First, preheating the combustion air to the levels requiredto enhance the kinetics requires several modifications to both thesystem and the air piping. The air heater and economizer sections mustbe modified to allow the incoming air to be heated to highertemperatures, which may require modifications to the rest of the steamcycle components. The ductwork and windbox, as well as the burneritself, must also be modified to handle the hot air. All of themodifications can be costly and can have a negative impact on theoperation of the boiler.

The primary barrier to the use of oxy-fuel firing in boilers has beenthe cost of oxygen. In order for the use of oxygen to be economic thefuel savings achieved by increasing the process efficiency must begreater than the cost of the supplied oxygen. For high temperatureoperations, such as furnaces without significant heat recovery, this iseasily achieved. However, for more efficient operations, such asboilers, the fuel savings attainable by using oxy-fuel firing istypically much lower than the cost of oxygen. For example, if a typicalcoal-fired utility boiler were converted from air firing to oxygenfiring, approximately 15 to 20% of the power output from that boilerwould be required to produce the necessary oxygen. Clearly, this isuneconomic for most boilers.

Thus there remains a need for a method for achieving reduced NOxemissions in combustion of fuel (particularly coal) containing one ormore nitrogenous compounds and especially for a method which can becarried out in existing furnaces without requiring extensive structuralmodifications.

BRIEF SUMMARY OF THE INVENTION

One aspect of the present invention, which can also be considered amethod for retrofitting existing combustion devices, is a method thatreduces the amount of NOx emitted comprising:

providing a combustion device that has a primary combustion zone and aburn out zone;

feeding air, and nonaqueous fuel that contains bound nitrogen and isselected from the group consisting of atomized hydrocarbon liquid andpulverulent hydrocarbon solids, through a burner into said primarycombustion zone; and

combusting the fuel in a flame in the primary combustion zone that has afuel-rich zone, while

feeding oxygen into said fuel by injecting it directly into said fuel insaid primary combustion zone as said fuel emerges from said burner or byadding it to the air that is fed through said burner, so that the oxygencombusts with said fuel in said fuel-rich zone, in an amount of saidoxygen which is less than 20% or even than 25% of the stoichiometricamount required for complete combustion of said fuel, and adjusting theamount of air fed through said burner so that the stoichiometric ratioin said primary combustion zone is between 0.6 and 0.99,

and adding air into said burn out zone from a source other than saidburner in an amount containing sufficient oxygen that the total amountof oxygen fed into said device is at least the stoichiometric amountneeded for complete combustion of said fuel, and combusting residualcombustibles from said primary combustion zone in said burn out zone.Preferably this stoichiometric ratio represents a variation of not morethan 10% from the stoichiometric ratio in the primary combustion zonecompared to the stoichiometric ratio without said addition of oxygen.

Another aspect of the present invention can be considered a method foroperating a combustion device, whether retrofitted or constructed newembodying the features of this invention, wherein the method reduces theamount of NOx emitted, comprising:

providing a combustion device;

feeding air, and nonaqueous fuel that contains bound nitrogen and isselected from the group consisting of atomized hydrocarbon liquid andpulverulent hydrocarbon solids, through an aerodynamically staged burnerinto said device; and

combusting said fuel in a flame that contains a fuel-rich zone, while

feeding oxygen into said fuel by injecting it directly into said fuel insaid fuel-rich zone as said fuel emerges from said burner or by addingit to the air that is fed through said burner, so that the oxygencombusts with said fuel in said fuel-rich zone, in an amount of saidoxygen which is less than 20% or even than 25% of the stoichiometricamount required for complete combustion of said fuel, and adjusting theamount of air fed through said burner so that the stoichiometric ratioin said fuel rich zone is between 0.1 and 0.85, while maintaining orenlarging the size of said fuel-rich zone compared to its size whencombustion is carried out in said combustion device without said oxygenfeeding step but under otherwise identical conditions.

As used herein the term “stoichiometric ratio” when used in the contextof an oxidant stream containing oxygen and a fuel stream smeans theratio of oxygen in the oxidant stream and the fuel stream to the totalamount of oxygen that would be necessary to convert fully all carbon,sulfur and hydrogen present in the substances comprising the fuel streaminto carbon dioxide, sulfur dioxide and water.

As used herein, the term “fuel-rich” means having a stoichiometric ratioless than 1.0 and the term “fuel lean” means having a stoichiometricratio greater than 1.0.

As used herein, the term “bound nitrogen” means nitrogen present in amolecule other than as N₂.

As used herein, “nonaqueous” means not suspended in, dissolved in, ordispersed in water, and not containing water, except that it does notexclude adsorbed water or water of hydration.

As used herein, the term “primary combustion zone” means the regionwithin a combustion device immediately adjacent the burner outlets andwhich is mostly occupied by the flame or flames from the burner orburners.

As used herein, the term “burn out zone” means the region within acombustion device that is between the primary combustion zone and theflue, outside the flame or flames that are in the primary combustionzone, where overfire air is injected and the residual fuels andcombustibles from the primary combustion zone are burned with overfireair.

As used herein, the term “primary combustion air” means air that hasalready been commingled with fuel as the fuel and this air are fed intoa combustion device, e.g. through an orifice of a burner.

As used herein, the term “secondary combustion air” means air that isfed into a combustion device through one or more orifices of a burner,but which has not been commingled with fuel as this air is fed into thecombustion device.

A burner that has orifices for secondary air may have additionalorifices for feeding air which additional orifices are further from thepoint of entry of the fuel through the burner than are the orifices forthe secondary air. As used herein, the term “tertiary combustion air”means air that is fed into a combustion device through such additionalorifices. If a burner also has orifices positioned even further from thepoint of entry of the fuel than the orifices for the tertiary air, thenair fed through such further orifices is termed herein “quaternarycombustion air”.

As used herein, the term “aerodynamically staged burner” means a burnerin which all of the combustion air is introduced from the same burnerport or adjacent to the burner port, and is capable of operating underconditions in which air velocities and flow patterns are present thatcreate a relatively large fuel rich first zone along the axis of theburner, followed by relatively gradual mixing of secondary and tertiaryair along the length of the furnace.

As used herein, the term “over fire air” (or “OFA”) means air which isinjected into a combustion device separately from the burner or burnersin the combustion device to provide a large fuel rich primary combustionzone and a burnout zone where combustion is completed by mixing OFA withthe unburned fuel and the products of partial combustion from theprimary combustion zone.

References herein to feeding “oxygen”, to the “oxygen” that is fed, andother references herein to the use of “oxygen” in an analogous context,mean gaseous streams that contain at least 35 vol. % O₂. Preferably,oxygen is provided as a gaseous stream containing at least 50 vol. % O₂,more preferably containing at least 80 vol. % O₂, and even morepreferably containing at least 90 vol. % O₂. It should also beunderstood that references herein to combustion or reaction involving“oxygen” refer to O₂ itself.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional representation of one embodiment ofapparatus for carrying out the present invention.

FIG. 2 is a cross-sectional representation of a burner useful forcarrying out the present invention.

FIGS. 3 a–3 d are cross-sectional representations of lances useful forfeeding oxygen into burners in accordance with the present invention.

FIG. 4 is laboratory scale test results showing reduction of NOxemissions with the present invention.

FIG. 5 is pilot scale low NOx burner test results showing reduction ofNOx emissions with the present invention.

FIG. 6 is commercial scale low NOx burner test results showing reductionof NOx emissions with the present invention.

FIG. 7A is a cross-section view of another type of boiler furnace withwhich the present invention can be utilized, wherein fuel and oxidantare fed from separate ports tangentially into the furnace. FIG. 7B is atop view of the furnace depicted in FIG. 7A, showing the tangential flowof fuel and oxidant into the furnace. FIG. 7C is a front view frominside the furnace looking at the fronts of the ports.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described with reference to the Figures, althougha description that refers to the Figures is not intended to limit thescope of that which is considered to be the present invention.

FIG. 1 shows combustion device 1, which can be any apparatus whereincombustion is carried out in the interior 2 of the device. Preferredcombustion devices include furnaces and boilers which are used togenerate steam to generate electric power by conventional means, notshown.

Each burner 3 in a sidewall or end wall of combustion device 1 feedsfuel, air and oxygen from sources thereof outside the combustion device1 into the interior 2 of combustion device 1. Suitable fuels includehydrocarbon liquids, such as fuel oil, and also include pulverulenthydrocarbon solids, a preferred example of which is pulverized coal orpetroleum coke.

As seen in FIG. 1 and more closely in FIG. 2, burner 3 is preferablycomprised of several concentrically arranged passages, although otherconstructions to the same effect can be used. The fuel is fed intocombustion device 1 through annular passage 4, disposed concentricallyaround lance 5 through which oxygen is fed as described herein.Preferably, the fuel is transported from a supply source 20 to one ormore burners 3 and propelled through burner 3 into the interior 2 ofcombustion device 1, by suitable pump means in the case of liquids suchas fuel oil, and by blowers and impellers of conventional design in thecase of hydrocarbon solids such as pulverized coal, which areconventionally fed into the combustion device with the aid of transportair (which is the primary combustion air). Liquid hydrocarbon fuels arepreferably fed through one or more atomizing nozzles of conventionaldesign, to feed the liquid fuel into the combustion chamber as discrete,dispersed droplets with atomizing air. An effective amount typicallyabout 1.5 to 2.0 lb of primary air is used to transport 1 lb of coal,which corresponds to about 20% of the stoichiometric combustion airrequired for complete combustion of bituminous coal. For combustion ofheavy oil about 0.5 to 1.0 lb of primary air is used to atomize 1 lb ofoil.

Referring to FIG. 2, combustion air 22 is supplied by an FD fan to oneor more windboxes 21 and fed to air passages of one or more burners 3.Secondary combustion air 15 is fed through burner 3 into combustiondevice 1, preferably through concentrically arranged annular passages 11surrounding the annular space 4 through which the hydrocarbon fuel isfed. Preferably tertiary combustion air 16 is fed through burner 3 intocombustion device 1, preferably through concentrically arranged annularpassages 12 surrounding the secondary air passage. Preferably combustionair is also fed through over fire air port 7 (seen in FIG. 1) intocombustion device 1. Preferably, the oxygen is fed into the interior 2of the device apart from any secondary and tertiary combustion air. Thatis, the oxygen that is fed through burner 3 in accordance with thisinvention is preferably completely consumed in combustion with the fuel,before that oxygen has an opportunity to become commingled withsecondary and tertiary combustion air before or immediately after it isfed into combustion device 1, especially when no over fire air is used.Alternatively, referring still to FIG. 2, the fuel could be fed throughannular passage 4, and the oxygen fed through lance 5 surrounded byannular passage 4 or the oxygen could be fed through passage 11surrounding annular passage 4.

Preferred low NOx burners have primary (fuel feeding), secondary andtertiary air passages for good aerodynamic adjustability. However, otherlow NOx burner designs using only primary and secondary air feeds can beused. Once the optimum settings with the three passages have beendetermined, the secondary air swirl vanes and passage can be designed tocreate about the same aerodynamic mixing characteristics as with thethree-passage design. Alternatively, burners with an additional(quaternary) passage can be used (such as the RSFC™ burner described inU.S. Pat. No. 5,960,724).

Before a combustion device is retrofitted in accordance with the presentinvention to reduce the formation of NOx formed in the operation of thecombustion device, lance 5 for feeding oxygen is not yet present.Combustion is carried out between the hydrocarbon fuel and the oxygen inthe combustion air, resulting in formation of a flame 6. The region 8 ofthe flame closest to the end of burner 3, that is, where the hydrocarbonfuel emerges from the burner, is the fuel-rich zone. The area of theflame 6 around its periphery is relatively lean, as secondary andtertiary combustion air have not been fully mixed or reacted with fuel.When the amount of combustion air 22 to burner 3 is reduced and asufficient amount of air is fed from over fire air port 7 for globalcombustion staging, the entire lower zone of the furnace, or primarycombustion zone (PCZ) 10, below over fire air port 7 becomes fuel rich,except the areas near burners 3 where air is injected and not yet fullymixed or reacted with fuel.

Then, in the implementation of the present invention, lance 5 is added.Alternatively, a burner that feeds fuel and combustion air is replacedwith a burner that performs as shown in the Figures.

Preferably, air is also fed through over fire air port opening 7 intothe interior of combustion device 1, to make the primary combustion zone10 less fuel lean or more fuel rich and to provide additional oxygenhelping to achieve complete combustion of the fuel in the burnout zone9. The oxygen in the combustion air fed through burner 3, combined withthe oxygen contained in air fed at opening 7, if used, are sufficient toenable complete combustion of the fuel, and typically contain 10 to 25volume percent excess oxygen over the amount required for the completecombustion of the fuel.

Preferably, the secondary and tertiary combustion air are fed at theburner 3 so as to swirl about a longitudinal axis, thereby creating arecirculation zone near each burner and improving commingling of air andfuel. Swirl can be achieved by known techniques, such as providingdeflectors, 13 and 14, in the annular passages for secondary andtertiary air flow of the burner which direct the flow of the streams inthe desired swirling direction. It is preferred to provide a high degreeof swirl, preferably a swirl number, as defined in “CombustionAerodynamics”, J. M. Beer and N. A. Chigier, Robert E. KriegerPublishing Company, Inc., 1983, of 0.6 to 2.0.

In the practice of this invention with over fire air, it is preferredthat the total amount of air fed through burner 3, i.e., the sum ofprimary, secondary and tertiary air, is between 60 and 99% of thestoichiometric air requirement for complete combustion. Most preferablythe total amount of air fed through burner 3 into the primary combustionzone is about 70 to 85% of the stoichiometric air requirement forcomplete combustion.

The velocity of each stream of primary, secondary and tertiarycombustion air is preferably 50 to 150 feet per second at the exit ofthe nozzle from which the air emerges. The velocity of the oxygeninjected through lance 5, at the exit of the nozzle from which theoxygen emerges, is preferably within 10% to 900%, more preferably within25% to 400% of the velocity of the primary air.

Tests have suggested that a preferred approach is to expose at leastsome of the fuel particles or droplets to a high concentration of oxygenas opposed to uniformly enriching the overall combustion air. The simpleapproach of injecting oxygen into the windbox 21 of a low NOx burnersuch that the enriched air is fed to the entire burner, including thecritical primary stage air, is not considered as effective.

When pure oxygen is premixed or mixed rapidly into the coal transportstream (primary air stream) using 20% of stoichiometric air and theoverall combustion stoichiometric ratio is kept constant at 1.15 bytaking out the stoichiometrically equivalent amount of air from eithersecondary or tertiary air (*), the following average concentrations ofoxygen in the transport air stream and in the overall combustion air arecalculated, assuming the air is dry and contains 21% O₂.

% of Avg. O₂ stoichiometric air O₂ concentration concentration inreplaced in transport total combustion with O₂ (*) air (vol. %) air(vol. %) 0 21.0 21.0 5 24.9 21.7 10 28.5 22.5 15 31.7 23.4 20 34.7 24.325 37.4 25.4 (* e.g. 5 cf of air replaced with 1.05 cf of pure O₂ togive the same amount of O₂)

In this example, due to the small amount of oxygen used, only modestincreases in the oxygen concentration of air are achieved when mixeduniformly even when oxygen is mixed only with the transport air. Apreferred method is to inject oxygen into the coal/air transport streamat the tip of the nozzle of the lance. In this case some of the coalparticles are mixed with oxygen jets and locally create zones of coalhigh O₂ mixture. Such conditions may provide zones of rapid ignitionsources and facilitate early ignition and devolatilization as comparedto the case oxygen is premixed with the transport air stream.

Another preferred method is to inject oxygen from the inner or outerannular space adjacent to the coal stream. In this case the favorableoxygen rich combustion condition is provided at the boundary of the coaland oxygen streams.

When oxygen is injected separately at high velocity parallel to the fuelstream, as was the case for Farmayan, et al., the oxygen jet(s) may bediluted quickly with surrounding gases and its effectiveness may beretarded. Thus, the method of oxygen injection has to be carefullydesigned.

The present invention improves, that is, lessens, the formation of NOxin the combustion device by feeding oxygen into the entering hydrocarbonfuel stream as described herein. More specifically and preferably, theoxygen is fed as a concentrated oxygen stream comprising preferably atleast 50 vol. % O₂, more preferably at least 80 vol. % O₂, mostpreferably at least 90 vol. % O₂ and is fed directly into thehydrocarbon fuel as it emerges from the burner and enters the interior 2of combustion device 1. Thus, at least some of the particles of solidfuel, or the droplets of liquid fuel, as the case may be, enter thecombustion device and the fuel-rich zone of flame 6, in a gaseousatmosphere containing a high concentration of oxygen.

When over fire air is used for global combustion staging, preferablywith air burners equipped with three or four separate air passages,oxygen may be premixed with the primary or secondary air or both, usingsuitable spargers within the gas passages in burner 3.

The oxygen is preferably fed through a lance 5 or similar feed line thatcan be open at the end that opens into combustion device 1, or that isclosed at the end and has two or more openings in its periphery adjacentthat closed end, such that oxygen flows out through those openingsdirectly into the hydrocarbon fuel entering the combustion device fromthe burner.

Referring to FIGS. 7A and 7C, a tangentially fired furnace 1 comprisesan array of ports for injecting fuel, and ports for injecting combustionair, into the furnace interior. Typically the fuel ports and thecombustion air ports are arrayed in a vertical row, alternating witheach other, as is illustrated in FIGS. 7A and 7C wherein ports 31 forinjecting fuel alternate with ports 32 for injecting combustion air. Thefuel combusts in the furnace interior with the combustion air. Thefurnace is also equipped with overfire air ports 7.

The present invention is readily adapted to furnaces having this type ofconstruction, for instance by providing a lance 5 in one or more of thefuel ports and then feeding oxidant in the required amounts as taughtherein into the fuel as it emerges from the burner. Oxygen lances 5 canalso be placed in one or more of the combustion air ports or outside ofthe air and fuel ports, and oxygen is injected from the lance(s) towardthe adjacent fuel stream.

FIGS. 3 a through 3 d show various lance configurations that can beemployed. Other lance configurations can be used. In FIG. 3 a, lance 5ends with a single orifice 31 that is preferably oriented along the axisof the lance.

In FIG. 3 b, the end of lance 5 is closed and two or more, preferablytwo to sixteen, more preferably four to eight nozzles 32 along theperimeter of the lance near the hot end of the lance are provided forradial oxygen injection. More than one row of radial nozzles can also beprovided along the perimeter of the lance near the hot end. One to fouror more nozzles can also be provided in the end of this lance.

In FIG. 3 c, two or more and preferably two to sixteen, more preferablyfour to eight nozzles 32 are provided radially near the closeddownstream end of the lance 5, and two or more, preferably two tosixteen, preferably four to eight nozzles 33 are provided each of whichforms an angle greater than 0 degrees and less than 90 degrees to theaxis of the direction of flow of oxygen into the lance 5.

In FIG. 3 d, two or more and preferably two to eight nozzles 34 areprovided along the perimeter of the lance 5 near the hot end of lance 5,each of which forms an angle of 30 to 90 degrees, preferably an angle of30 to 60 degerees, with respect to the reverse of the direction of flowof oxygen into the lance 5.

In these and other lance embodiments the nozzles through the side of thelance can be arrayed on one or more than one circumference.

The optimal angle of oxygen injection for NOx control depends on thetangential and radial momentum of the surrounding air, the burner portgeometry and the nature of the burner airflow pattern near the oxygenlance. Accordingly, for better results in burners having low radial airmomentum, the optimal angle is 90° or greater from the axis of theburner, whereas obtaining better results in burners having higher radialmomentum will generally require the angle to be reduced to avoid mixingthe oxygen with the air stream. With highly radial air flow the optimalangle is 15° or less (largely axial injection). For those burners thatuse techniques that create a strong air flow component in the radialdirection, such as high swirl with a shallow diverging burner port orair deflectors, oxygen nozzles that are primarily angled in the axialdirection (angle of less than 30° from the axis) are optimal. Forburners in which the air flow is predominantly axial (that is, theradial component of the air flow is small or non-existent) it ispreferred to inject the oxygen in the radial direction (angles between45° and 135° from the axial flow component).

When oxygen is injected into combustion device 1 as described herein,the flow rate of combustion air fed through burner 3 is simultaneouslyreduced to maintain or reduce the primary combustion zone stoichiometricratio. When overfire air is used, the primary combustion zonestoichiometric ratio with oxygen injection is preferably between 60 and99%, more preferably between 60 to 85%, %, most preferably between 70 to85%, of the stoichiometric air requirement for complete combustion. Theamount of oxygen fed in this manner should be sufficient to establish astoichiometric ratio in the fuel-rich zone 8 of flame 6 which is lessthan about 0.85 and is preferably much less than 0.85, e.g. 0.65 orless. The amount of oxygen fed through line 5 should be less than 20% ofthe stoichiometric amount required for the complete combustion of thefuel. Preferably, the amount corresponds to less than 15% of thestoichiometric amount required for complete combustion of the fuel. Morepreferably, the amount corresponds to less than 10% of thestoichiometric amount required for complete combustion of the fuel. Mostpreferably, the amount corresponds to less than 5% of the stoichiometricamount required for complete combustion of, the fuel.

Contrary to the present invention, U.S. Pat. No. 4,495,874 disclosesincreased NO with 2% oxygen enrichment, which as defined in that patentis equivalent to about 13% of the stoichiometric amount required forcomplete combustion of the fuel. It also discloses sharply decreased NOat higher enrichments (4% enrichment, which as defined in that patent isequivalent to about 23% of the stoichiometric amount required forcomplete combustion of the fuel. The patent clearly does not anticipatethe findings of the present invention that a small amount of oxygen,i.e., less than 20% of the stoichiometric amount required for thecomplete combustion of the fuel, can unexpectedly reduce NOx emissionswhen oxygen or oxygen enriched air is injected in the fuel rich zone ofthe primary combustion zone.

NOx emission strongly depends on the local stoichiometric conditions. Asinjection of oxygen makes the local stoichiometric condition leaner, onehas to consider the change in the local stoichiometric conditions afterthe oxygen injection. For example, injection of oxygen, equivalent to10% of the stoichiometric air, into a locally fuel rich zone at astoichiometric ratio of 0.4 (SR=0.4), without changing the flow rate ofcombustion air being fed, would alter the local stoichiometricconditions to SR=0.5 and would be expected to decrease NOx emissionssubstantially. However, this is because SR=0.4 is too fuel rich foroptimum NOx reduction. Such an effect is much greater than that from“replacing 10% air with oxygen” while keeping the local stoichiometriccondition constant at SR=0.4. If the same amount of oxygen is injectedinto the fuel rich combustion zone, without changing the flow rate ofthe combustion air, where the local stoichiometric condition is SR=0.95,NOx emission is expected to increase sharply as the local stoichiometriccondition is increased to SR=1.05.

Thus, it is generally preferred to inject oxygen into the richest areaof the flame. In a combustion device using aerodynamically stagedburners, the stoichiometric ratio in the fuel rich zone of the flamewith oxygen injection is between 0.1 and 0.85, preferably between 0.4and 0.75.

Injection or mixing of oxygen into the tertiary air and quaternary, ifused, should be avoided in an aerodynamically staged burner without OFA.This is because tertiary and quaternary air is mixed in the relativelylean area of a flame. If the low Nox burner has only primary andsecondary air, injection or mixing of oxygen into the secondary airshould be avoided. In theory the optimization of local stoichiometriccondition can be done with any oxidants including air. However, oxygenis more effective because only a small volume is required and localstoichiometric condition can be changed without a large impact on theoverall aerodynamic mixing conditions of the flame.

Another important requirement is that oxygen enrichment has to be donein such a way as to preserve or enhance the physical size of the fuelrich zone (the “N₂ forming zone”) of an aerodynamically staged flame.The method of oxygen injection and the consequent reduction of air flowsin certain air passages of a burner would influence the aerodynamicstaging conditions of the burner, and hence the physical size and thelocal stoichiometric conditions. If the size of the fuel rich zone isreduced and the average gas residence time in the fuel rich zone isreduced as a result of oxygen injection, such a change could cause NOxincreases. For example, high velocity injection of oxygen through anaxial lance such as the one shown in FIG. 3 a would effectively increasethe axial momentum of the surrounding coal/air stream, which in turn mayenhance the mixing with secondary and tertiary air. As a result the sizeof the fuel rich NOx reduction zone of the flame may be reduced and NOxmay increase. On the other hand when the oxygen flow is injectedradially from an axially located oxygen lance such as the one shown inFIG. 3 b near the tip of the burner, it may effectively increase therecirculation zone near the burner and hence increase the size of thefuel rich zone and further promote NOx reduction by oxygen enrichment.Complex impacts of oxygen injection on the burner aerodynamic conditionshave to be evaluated carefully for a specific burner to achieve NOxreduction.

Without intending to be bound by any particular explanation of theunexpected performance of this invention, the performance of thecombustion device operated in accordance with this invention isconsistent with a mechanism in which the injected oxygen causes anincrease in the temperature of that portion of the flame closest to theburner, which in turn causes relatively volatile components present inthe hydrocarbon fuel to enter the gas phase from the fuel and undergopartial reaction with the ambient oxygen, thereby creating a relativelyreducing atmosphere that enables nitrogen-containing species releasedfrom the combusting fuel to be converted to molecular nitrogen, that is,N₂, rather that converted to NOx and other nitrogenous compounds such asHCN and NH₃.

Typically, the temperature of the fuel-rich zone into which the fuel andthe oxygen enter is on the order of 2500° F. or higher. Feeding theoxygen in this manner can cause the base of flame 6 to draw nearer tothe opening of burner 3, or even to become attached to burner 3.However, feeding the oxygen in the manner described herein into thehydrocarbon fuel as it emerges from the burner proceeds in the samemanner, even if the flame becomes attached to the burner. In steadystate operation, for instance after a combustion device has beenretrofitted in accordance with the teachings herein, operation of thecombustion device continues on the basis that less than 20%, preferablyless than 15%, more preferably less than 10%, most preferably less than5%, of the stoichiometric amount of oxygen required for the completecombustion of the fuel is fed into the fuel, while combustion air is fedthrough the burner in an amount less than otherwise would be the case,so that the total amount of oxygen fed into the device is at least thestoichiometric amount needed for complete combustion of the fuel.

Operation of a combustion device, fueled by hydrocarbon liquid fuel orpulverulent solid hydrocarbon fuel, in accordance with the teachingsherein, has been found to produce surprising and significant reductionin the amount of NOx formed by combustion in the combustion device.

EXAMPLE 1

Oxygen was mixed into the combustion air supplied to the first stage ofa 17 kW thermal self-sustained downfired experimental furnace with 6″internal diameter. The oxygen enriched combustion air was fed to apremixed non-swirled pulverized coal-air burner. High volatile Abituminous coal (Illinois No.6) was used. Oxygen was fed at a rate toreplace 20% by volume of the combustion air, on an oxygen equivalentbasis, supplied to the primary combustion zone (PCZ). This wasequivalent to 10 to 20% of the stoichiometric air requirement. “Overfire air” was injected into the furnace approximately eight feetdownstream of the burner face, which provided gas residence time ofabout 1 second in the PCZ. Sufficient over fire air was injected to keepthe overall stoichiometric ratio at approximately 1.2. Although thetemperature at the over fire air injection point varied according to theprimary combustion zone stoichiometric ratio, it was held as constantbetween the air (baseline) and oxygen enriched cases. As can be seenfrom FIG. 4 addition of small amounts of oxygen reduced the emission ofnitrogen oxides when the first stage was operated at below astoichiometric ratio of about 0.9.

EXAMPLE 2

A low NOx coal-air burner was fired at about 4 MMBtu/hr in a refractorylined test furnace with internal dimensions of about 3.6 ft wide×3.6 fthigh and 41 ft long. A pair of over fire air ports were located at about10.5 ft from the burner exit. The burner is similar to one shown in FIG.2 and consists of a central round passage and several annular passagesfor coal, air, oxygen and natural gas streams. The central passage waseither used to insert an 1.9″ OD and 1.5″ ID oxygen lance or blocked toprovide a bluff body to enhance gas recirculation for flame stability.Coal and primary air were injected from the first annular passage of3.068″ OD and 1.9″ ID. The second annular passage (4.026″ OD and 3.5″ID) was used to inject either natural gas or oxygen. The third (6.065″OD and 4.5″ ID) and the fourth (7.981″ OD and 6.625″ ID) annularpassages were used for secondary and tertiary air flows and wereequipped with variable swirl generators to impart swirling flows. Theburner is designed to provide an aerodynamically staged combustioncondition. The axial velocities of the primary air and secondary air aresimilar to provide a slow mixing of the secondary air with the coalstream. The tertiary air has a significantly higher velocity than thatof the secondary air. Thus, the secondary air provides a “buffer” formixing between the tertiary air and the coal stream. A relatively largefuel rich combustion zone is created along the axis of the burner withrelatively gradual mixing of secondary and tertiary air along the lengthof the furnace.

FIG. 5 shows the results of NOx emissions measured under differentoxygen injection methods. Pure oxygen was injected through a cylindricallance located in the axis of the burner. Different nozzle designs wereused to inject oxygen and to mix with the adjacent annular coal stream.The amount of oxygen injected ranged from 5 to 15% of the stoichiometricoxygen. When oxygen was injected, the stoichiometrically equivalentamount of air was taken out of the secondary and tertiary air streams soas to maintain the same primary combustion zone and overall combustionstoichiometric ratio (fixed at SR=1.15). The primary air flow rate waskept constant at about SR=0.15. The over fire air for global combustionstaging was injected perpendicular to the axis of the furnace from twodirectly opposed air nozzles.

Below a primary combustion zone SR (the stoichiometric ratio of theprimary combustion zone) of about 0.80, significant NOx reduction, ascompared with the air baseline, was obtained regardless of the type ofoxygen nozzles used. At higher burner SR's NOx emissions were higherwhen a reverse angle nozzle, which has eight ¼ inch diameter holes asshown in FIG. 3.d (also referred to herein as a Type B nozzle) was usedand about the same or lower with a nozzle of the type shown in FIG. 3.c,which has eight ¼ inch diameter radial holes and four ¼ inch diameterforward angles holes (also referred to herein as a Type A nozzle),depending on the amount of oxygen injected.

The observed results can be explained in terms of the changes caused bythe oxygen injection in the aerodynamic staging conditions of theburner, i.e., the physical size of the fuel rich zone and the localstoichiometric conditions. When the primary combustion zone is deeplystaged (SR less than 0.8), a large volume of the furnace space betweenthe burner and the staging air injection point is maintained fuel rich.Although the injection of oxygen and reduction of secondary and tertiaryair flows would change the local stoichiometric conditions near theburner (and either increase or decrease the volume of the fuel rich zonenear the burner), the bulk of the primary combustion zone remains asfuel rich. The main effect of oxygen is to increase the temperature andaccelerate the kinetics of NOx reduction in most of the primarycombustion zone. Since the volume of the large fuel rich zone is littlechanged, significant NOx reduction is achieved relatively independent ofthe oxygen nozzle type.

As the amount of the over fire air is reduced, the size of the fuel richzone is progressively reduced. At a burner SR=1.15, no staging air isused and the fuel rich zone is created solely by aerodynamic staging ofthe burner. The injection of oxygen would affect the mixing pattern ofsecondary and tertiary air flows with the coal stream and the size ofthe fuel rich zone could change significantly. If the size of the fuelrich zone is reduced and the average gas residence time in the fuel richzone is reduced as a result of oxygen injection, such a change couldcause NOx increases.

NOx emissions at SR=1.15, i.e., without staging air, were very sensitiveto the O₂ nozzle types and the amount of oxygen used. CFD studies showedthe following mixing conditions with a nozzle shown in FIG. 3 b witheight ¼ inch radial holes. The radial oxygen jets penetrate into theannular coal stream for a short distance and mix rapidly with thesurrounding coal stream. The annular coal stream partially flow inbetween the “fingers” of oxygen jets and partially expands radially. Asa result, the diameter of the recirculation zone near the burner isincreased, causing the size of the fuel rich zone to expand.

With a Type B O₂ nozzle the upstream angle of the oxygen jets isbelieved to have caused significant mixing of the primary coal streamand the tertiary, resulting in NOx increase. In general higher O₂ flowfrom radial or angled radial nozzles increases the oxygen jet velocityand causes more mixing. Thus, the size and angle of oxygen nozzles haveto be carefully designed to rapidly mix oxygen into the coal stream, yetnot to cause too much mixing between the tertiary air and the coalstream.

EXAMPLE 3

A commercial low NOx coal-air burner, the RSFC™ burner described in U.S.Pat. No. 5,960,724, was fired at about 24 MMBtu/hr in a refractory linedtest furnace with internal dimensions of about 7.5 ft wide×7.5 high and34 ft long. One or two opposed pairs of over fire air ports were locatedat about 26 feet from the burner exit. The burner consists of a centralround passage and several annular passages for coal, air, oxygenstreams. The central passage was used to insert an 1.9″ OD and 1.5″ IDoxygen lance. Coal and primary air were injected from the first annularpassage. The second, third and the fourth annular passages were used forsecondary, tertiary, and quaternary air flows and were equipped withvariable swirl generators to impart swirling flows. The burner isdesigned to provide an aerodynamically staged combustion condition. Arelatively large fuel rich combustion zone is created along the axis ofthe burner with relatively gradual mixing of tertiary and quaternary airalong the length of the furnace.

Oxygen was injected through a round lance located in the axis of theburner. The nozzle design similar to the one shown in FIG. 3 c, whichhas eight ⅜ inch diameter radial holes and four ⅜ inch diameter axiallyoriented holes was used to inject oxygen and to mix with the adjacentannular coal stream. The amount of oxygen injected ranged form 5 to 15%of the stoichiometric oxygen. When oxygen was injected, thestoichiometrically equivalent amount of air was taken out of thesecondary, tertiary and quaternary air streams so as to maintain thesame primary combustion zone (SR=0.75) and overall combustionstoichiometric ratio (SR=1.15). The primary air flow rate was keptconstant at about SR=0.20. The over fire air for global combustionstaging was injected perpendicular to the axis of the furnace from twoto four directly opposed air ports.

The settings of adjustable swirl vanes for secondary, tertiary, andquaternary air flows were optimized to give lowest NOx emissions for aironly firing and the same settings were used when oxygen was injected.FIG. 6 shows the results of NOx emissions as a function of oxygeninjection, measured under three different test periods. Although thebaseline NOx emissions with air varied depending on test periods,significant NOx reductions were achieved by the present invention.

While the present invention has been described with principal referenceto wall-fired boilers such as the type illustrated in FIGS. 1 and 2,this description is not intended to suggest that the invention islimited in applicability to that type of combustion system. Theinvention is applicable to other systems wherein fuel and air arecombusted, including without limitation the tangentially-fired systemsof the type described with respect to FIGS. 7A–7C, and combustionsystems is known in the art as “cyclone” furnaces, wherein the primarycombustion zone of the furnace includes one or more enclosures eachhaving a cylindrical wall, a closed end wall, and an open end that opensinto the main chamber of the furnace through a wall of the furnace,wherein fuel, combustion air and oxidant (fed in the amounts as taughtherein into the fuel) are fed through the cylindrical wall and the endwall into the enclosure in a direction such that they rotate around thecentral axis of rotation of the enclosure and combust to form a flameand heat of combustion which are emitted through the open end into themain chamber of the furnace.

Other types of burners can be employed in addition to those exemplifiedherein, such as so-called split-stream burners wherein the stream offuel is split into a plurality of streams separated from each other, andeven diverging from each other, as the fuel enters the combustionchamber. With this type of burner, the oxygen is fed from acorresponding plurality of lances into each stream of fuel, or from alance with a plurality of nozzles oriented toward each stream of fuel,and the stoichiometric requirements of oxygen are based on the totalamounts of fuel and oxygen being fed.

1. A combustion method that reduces the amount of NOx emittedcomprising: providing a combustion device that has a primary combustionzone and a burn out zone; feeding air, and nonaqueous fuel that containsbound nitrogen and is selected from the group consisting of atomizedhydrocarbon liquid and pulverulent hydrocarbon solids, through a burnerinto said primary combustion zone; and combusting the fuel in a flame inthe primary combustion zone that has a fuel-rich zone, while feedingoxygen into said fuel by injecting it directly into said fuel in saidprimary combustion zone as said fuel emerges from said burner or byadding it to the air that is fed through said burner, so that the oxygencombusts with said fuel in said fuel-rich zone, in an amount of saidoxygen which is less than 20% of the stoichiometric amount required forcomplete combustion of said fuel, and adjusting the amount of air fedthrough said burner so that the stoichiometric ratio in said primarycombustion zone is between 0.6 and 0.99, and adding air into said burnout zone from a source other than said burner in an amount containingsufficient oxygen that the total amount of oxygen fed into said deviceis at least the stoichiometric amount needed for complete combustion ofsaid fuel, and combusting residual combustibles from said primarycombustion zone in said burn out zone.
 2. A method according to claim 1wherein a stream of fuel is fed through said burner and oxygen is fedinto said fuel by injecting it through a hollow lance, positioned insaid stream, into the fuel as the fuel emerges from the burner.
 3. Amethod according to claim 1 wherein a stream of fuel is fed through anannular fuel passage of said burner, and oxygen is fed into said fuel byinjecting it through an annular passage surrounding or surrounded bysaid annular fuel passage.
 4. A method according to claim 1 wherein saidoxygen is injected directly into said fuel through a lance having atleast one orifice in the end of the lance, that is oriented along theaxis of the lance.
 5. A method according to claim 1 wherein said oxygenis injected directly into said fuel through a lance having a closed endand at least two nozzles along the perimeter of the lance near the endof the lance for radial oxygen injection.
 6. A method according to claim1 wherein said oxygen is injected directly into said fuel through alance having a closed end, at least two nozzles provided radially nearthe closed end, and at least two nozzles each of which forms an anglegreater than 0 degrees and less than 90 degrees to the axis of thedirection of flow of oxygen into the lance.
 7. A method according toclaim 1 wherein said oxygen is injected directly into said fuel througha lance having a closed end and having at least two nozzles providedalong the perimeter of the lance near the closed end of the lance, eachof which forms an angle of 30 to 90 degrees with respect to the reverseof the direction of flow of oxygen into the lance.
 8. A method accordingto claim 1 wherein said fuel is fed through said burner with air at avelocity of 50 to 150 feet per second, and oxygen is injected into saidfuel at a velocity 25% to 400% of the velocity of said air.
 9. A methodaccording to claim 1 wherein any secondary and tertiary air fed throughsaid burner has a swirl number of 0.6 to 2.0.
 10. A method according toclaim 1 wherein the stoichiometric ratio in said fuel rich zone isbetween 0.1 and 0.75.
 11. A method according to claim 1 wherein thestoichiometric ratio in said primary combustion zone is between 0.7 and0.85.
 12. A method according to claim 1 wherein the amount of saidoxygen fed into said fuel is less than 10% of the stoichiometric amountrequired for complete combustion of said fuel.
 13. A method according toclaim 1 wherein the amount of air fed through said burner is reduced byan amount containing sufficient oxygen that the primary combustion zonestoichiometric ratio varies by not more than 10% compared to thestoichiometric ratio without said addition of oxygen.
 14. A methodaccording to claim 1 wherein the air flow from said burner ispredominantly axial and the oxygen is injected into said fuel at anangle of 45 to 135 degrees from the longitudinal axis of the burner. 15.A method according to claim 14 wherein said angle is at least 90 degreesfrom the longitudinal axis of the burner.
 16. A method according toclaim 1 wherein the air flow from said burner is predominantly radialand the oxygen is injected into said fuel at an angle of 15 degrees orless from the longitudinal axis of the burner.
 17. A combustion methodthat reduces the amount of NOx emitted, comprising: providing acombustion device; feeding air, and nonaqueous fuel that contains boundnitrogen and is selected from the group consisting of atomizedhydrocarbon liquid and pulverulent hydrocarbon solids, through anaerodynamically staged burner into said device; and combusting said fuelin a flame that contains a fuel-rich zone, while feeding oxygen intosaid fuel by injecting it directly into said fuel in said fuel rich zoneas said fuel emerges from said burner or by adding it to the air that isfed through said burner, so that the oxygen combusts with said fuel insaid fuel-rich zone, in an amount of said oxygen which is less than 20%of the stoichiometric amount required for complete combustion of saidfuel, and adjusting the amount of air fed through said burner so thatthe stoichiometric ratio in said fuel rich zone is between 0.1 and 0.85,while maintaining or enlarging the size of said fuel-rich zone comparedto its size when combustion is carried out in said combustion devicewithout said oxygen feeding step but under otherwise identicalconditions wherein a stream of fuel is fed through said burner andoxygen is fed into said fuel by injecting it through a hollow lance,positioned in said stream, into the fuel as the fuel emerges from theburner.
 18. A method according to claim 17 wherein the stoichiometricratio in said fuel rich zone is between 0.4 and 0.75.
 19. A combustionmethod that reduces the amount of NOx emitted, comprising: providing acombustion device; feeding air, and nonaqueous fuel that contains boundnitrogen and is selected from the group consisting of atomizedhydrocarbon liquid and pulverulent hydrocarbon solids, through anaerodynamically staged burner into said device; and combusting said fuelin a flame that contains a fuel-rich zone, while feeding oxygen intosaid fuel by injecting it directly into said fuel in said fuel rich zoneas said fuel emerges from said burner or by adding it to the air that isfed through said burner, so that the oxygen combusts with said fuel insaid fuel-rich zone, in an amount of said oxygen which is less than 20%of the stoichiometric amount required for complete combustion of saidfuel, and adjusting the amount of air fed through said burner so thatthe stoichiometric ratio in said fuel rich zone is between 0.1 and 0.85,while maintaining or enlarging the size of said fuel-rich zone comparedto its size when combustion is carried out in said combustion devicewithout said oxygen feeding step but under otherwise identicalconditions wherein a stream of fuel is fed through an annular fuelpassage of said burner, and oxygen is fed into said fuel by injecting itthrough an annular passage surrounding or surrounded by said annularfuel passage.
 20. A method according to claim 19 wherein thestoichiometric ratio in said fuel rich zone is between 0.4 and 0.75. 21.A combustion method that reduces the amount of NOx emitted, comprising:providing a combustion device; feeding air, and nonaqueous fuel thatcontains bound nitrogen and is selected from the group consisting ofatomized hydrocarbon liquid and pulverulent hydrocarbon solids, throughan aerodynamically staged burner into said device; and combusting saidfuel in a flame that contains a fuel-rich zone, while feeding oxygeninto said fuel by injecting it directly into said fuel in said fuel richzone as said fuel emerges from said burner or by adding it to the airthat is fed through said burner, so that the oxygen combusts with saidfuel in said fuel-rich zone, in an amount of said oxygen which is lessthan 20% of the stoichiometric amount required for complete combustionof said fuel, and adjusting the amount of air fed through said burner sothat the stoichiometric ratio in said fuel rich zone is between 0.1 and0.85, while maintaining or enlarging the size of said fuel-rich zonecompared to its size when combustion is carried out in said combustiondevice without said oxygen feeding step but under otherwise identicalconditions wherein said oxygen is injected directly into said fuelthrough a lance having at least one orifice in the end of the lance,that is oriented along the axis of the lance.
 22. A method according toclaim 21 wherein the stoichiometric ratio in said fuel rich zone isbetween 0.4 and 0.75.
 23. A combustion method that reduces the amount ofNOx emitted, comprising: providing a combustion device; feeding air, andnonaqueous fuel that contains bound nitrogen and is selected from thegroup consisting of atomized hydrocarbon liquid and pulverulenthydrocarbon solids, through an aerodynamically staged burner into saiddevice; and combusting said fuel in a flame that contains a fuel-richzone, while feeding oxygen into said fuel by injecting it directly intosaid fuel in said fuel rich zone as said fuel emerges from said burneror by adding it to the air that is fed through said burner, so that theoxygen combusts with said fuel in said fuel-rich zone, in an amount ofsaid oxygen which is less than 20% of the stoichiometric amount requiredfor complete combustion of said fuel, and adjusting the amount of airfed through said burner so that the stoichiometric ratio in said fuelrich zone is between 0.1 and 0.85, while maintaining or enlarging thesize of said fuel-rich zone compared to its size when combustion iscarried out in said combustion device without said oxygen feeding stepbut under otherwise identical conditions wherein said oxygen is injecteddirectly into said fuel through a lance having a closed end and at leasttwo nozzles along the perimeter of the lance near the end of the lancefor radial oxygen injection.
 24. A method according to claim 23 whereinthe stoichiometric ratio in said fuel rich zone is between 0.4 and 0.75.25. A combustion method that reduces the amount of NOx emitted,comprising: providing a combustion device; feeding air, and nonaqueousfuel that contains bound nitrogen and is selected from the groupconsisting of atomized hydrocarbon liquid and pulverulent hydrocarbonsolids, through an aerodynamically staged burner into said device; andcombusting said fuel in a flame that contains a fuel-rich zone, whilefeeding oxygen into said fuel by injecting it directly into said fuel insaid fuel rich zone as said fuel emerges from said burner or by addingit to the air that is fed through said burner, so that the oxygencombusts with said fuel in said fuel-rich zone, in an amount of saidoxygen which is less than 20% of the stoichiometric amount required forcomplete combustion of said fuel, and adjusting the amount of air fedthrough said burner so that the stoichiometric ratio in said fuel richzone is between 0.1 and 0.85, while maintaining or enlarging the size ofsaid fuel-rich zone compared to its size when combustion is carried outin said combustion device without said oxygen feeding step but underotherwise identical conditions wherein said oxygen is injected directlyinto said fuel through a lance having a closed end, at least two nozzlesprovided radially near the closed end, and at least two nozzles each ofwhich forms an angle greater than 0 degrees and less than 90 degrees tothe axis of the direction of flow of oxygen into the lance.
 26. A methodaccording to claim 25 wherein the stoichiometric ratio in said fuel richzone is between 0.4 and 0.75.
 27. A combustion method that reduces theamount of NOx emitted, comprising: providing a combustion device;feeding air, and nonaqueous fuel that contains bound nitrogen and isselected from the group consisting of atomized hydrocarbon liquid andpulverulent hydrocarbon solids, through an aerodynamically staged burnerinto said device; and combusting said fuel in a flame that contains afuel-rich zone, while feeding oxygen into said fuel by injecting itdirectly into said fuel in said fuel rich zone as said fuel emerges fromsaid burner or by adding it to the air that is fed through said burner,so that the oxygen combusts with said fuel in said fuel-rich zone, in anamount of said oxygen which is less than 20% of the stoichiometricamount required for complete combustion of said fuel, and adjusting theamount of air fed through said burner so that the stoichiometric ratioin said fuel rich zone is between 0.1 and 0.85, while maintaining orenlarging the size of said fuel-rich zone compared to its size whencombustion is carried out in said combustion device without said oxygenfeeding step but under otherwise identical conditions wherein saidoxygen is injected directly into said fuel through a lance having aclosed end and having at least two nozzles provided along the perimeterof the lance near the closed end of the lance, each of which forms anangle of 30 to 90 degrees with respect to the reverse of the directionof flow of oxygen into the lance.
 28. A method according to claim 27wherein the stoichiometric ratio in said fuel rich zone is between 0.4and 0.75.
 29. A combustion method that reduces the amount of NOxemitted, comprising: providing a combustion device; feeding air, andnonaqueous fuel that contains bound nitrogen and is selected from thegroup consisting of atomized hydrocarbon liquid and pulverulenthydrocarbon solids, through an aerodynamically staged burner into saiddevice; and combusting said fuel in a flame that contains a fuel-richzone, while feeding oxygen into said fuel by injecting it directly intosaid fuel in said fuel rich zone as said fuel emerges from said burneror by adding it to the air that is fed through said burner, so that theoxygen combusts with said fuel in said fuel-rich zone, in an amount ofsaid oxygen which is less than 20% of the stoichiometric amount requiredfor complete combustion of said fuel, and adjusting the amount of airfed through said burner so that the stoichiometric ratio in said fuelrich zone is between 0.1 and 0.85, while maintaining or enlarging thesize of said fuel-rich zone compared to its size when combustion iscarried out in said combustion device without said oxygen feeding stepbut under otherwise identical conditions wherein said fuel is fedthrough said burner with air at a velocity of 50 to 150 feet per second,and oxygen is injected into said fuel at a velocity 25% to 400% of thevelocity of said air.
 30. A method according to claim 29 wherein thestoichiometric ratio in said fuel rich zone is between 0.4 and 0.75. 31.A combustion method that reduces the amount of NOx emitted, comprising:providing a combustion device; feeding air, and nonaqueous fuel thatcontains bound nitrogen and is selected from the group consisting ofatomized hydrocarbon liquid and pulverulent hydrocarbon solids, throughan aerodynamically staged burner into said device; and combusting saidfuel in a flame that contains a fuel-rich zone, while feeding oxygeninto said fuel by injecting it directly into said fuel in said fuel richzone as said fuel emerges from said burner or by adding it to the airthat is fed through said burner, so that the oxygen combusts with saidfuel in said fuel-rich zone, in an amount of said oxygen which is lessthan 20% of the stoichiometric amount required for complete combustionof said fuel, and adjusting the amount of air fed through said burner sothat the stoichiometric ratio in said fuel rich zone is between 0.1 and0.85, while maintaining or enlarging the size of said fuel-rich zonecompared to its size when combustion is carried out in said combustiondevice without said oxygen feeding step but under otherwise identicalconditions wherein any secondary and tertiary air fed through saidburner have a swirl number of 0.6 to 2.0.
 32. A method according toclaim 31 wherein the stoichiometric ratio in said fuel rich zone isbetween 0.4 and 0.75.
 33. A combustion method that reduces the amount ofNOx emitted, comprising: providing a combustion device; feeding air, andnonaqueous fuel that contains bound nitrogen and is selected from thegroup consisting of atomized hydrocarbon liquid and pulverulenthydrocarbon solids, through an aerodynamically staged burner into saiddevice; and combusting said fuel in a flame that contains a fuel-richzone, while feeding oxygen into said fuel by injecting it directly intosaid fuel in said fuel rich zone as said fuel emerges from said burneror by adding it to the air that is fed through said burner, so that theoxygen combusts with said fuel in said fuel-rich zone, in an amount ofsaid oxygen which is less than 20% of the stoichiometric amount requiredfor complete combustion of said fuel, and adjusting the amount of airfed through said burner so that the stoichiometric ratio in said fuelrich zone is between 0.1 and 0.85, while maintaining or enlarging thesize of said fuel-rich zone compared to its size when combustion iscarried out in said combustion device without said oxygen feeding stepbut under otherwise identical conditions wherein the amount of saidoxygen fed into said fuel is less than 10% of the stoichiometric amountrequired for complete combustion of said fuel.
 34. A method according toclaim 33 wherein the stoichiometric ratio in said fuel rich zone isbetween 0.4 and 0.75.
 35. A combustion method that reduces the amount ofNOx emitted, comprising: providing a combustion device; feeding air, andnonaqueous fuel that contains bound nitrogen and is selected from thegroup consisting of atomized hydrocarbon liquid and pulverulenthydrocarbon solids, through an aerodynamically staged burner into saiddevice; and combusting said fuel in a flame that contains a fuel-richzone, while feeding oxygen into said fuel by injecting it directly intosaid fuel in said fuel rich zone as said fuel emerges from said burneror by adding it to the air that is fed through said burner, so that theoxygen combusts with said fuel in said fuel-rich zone, in an amount ofsaid oxygen which is less than 20% of the stoichiometric amount requiredfor complete combustion of said fuel, and adjusting the amount of airfed through said burner so that the stoichiometric ratio in said fuelrich zone is between 0.1 and 0.85, while maintaining or enlarging thesize of said fuel-rich zone compared to its size when combustion iscarried out in said combustion device without said oxygen feeding stepbut under otherwise identical conditions wherein the amount of air fedthrough said burner is reduced by an amount containing sufficient oxygenthat the primary combustion zone stoichiometric ratio varies by not morethan 10% compared to the stoichiometric ratio without said addition ofoxygen.
 36. A method according to claim 35 wherein the stoichiometricratio in said fuel rich zone is between 0.4 and 0.75.
 37. A combustionmethod that reduces the amount of NOx emitted, comprising: providing acombustion device; feeding air, and nonaqueous fuel that contains boundnitrogen and is selected from the group consisting of atomizedhydrocarbon liquid and pulverulent hydrocarbon solids, through anaerodynamically staged burner into said device; and combusting said fuelin a flame that contains a fuel-rich zone, while feeding oxygen intosaid fuel by injecting it directly into said fuel in said fuel rich zoneas said fuel emerges from said burner or by adding it to the air that isfed through said burner, so that the oxygen combusts with said fuel insaid fuel-rich zone, in an amount of said oxygen which is less than 20%of the stoichiometric amount required for complete combustion of saidfuel, and adjusting the amount of air fed through said burner so thatthe stoichiometric ratio in said fuel rich zone is between 0.1 and 0.85,while maintaining or enlarging the size of said fuel-rich zone comparedto its size when combustion is carried out in said combustion devicewithout said oxygen feeding step but under otherwise identicalconditions wherein the air flow from said burner is predominantly radialand the oxygen is injected into said fuel at an angle of 15 degrees orless from the longitudinal axis of the burner.
 38. A method according toclaim 37 wherein the stoichiometric ratio in said fuel rich zone isbetween 0.4 and 0.75.
 39. A combustion method that reduces the amount ofNOx emitted, comprising: providing a combustion device; feeding air, andnonaqueous fuel that contains bound nitrogen and is selected from thegroup consisting of atomized hydrocarbon liquid and pulverulenthydrocarbon solids, through an aerodynamically staged burner into saiddevice; and combusting said fuel in a flame that contains a fuel-richzone, while feeding oxygen into said fuel by injecting it directly intosaid fuel in said fuel rich zone as said fuel emerges from said burneror by adding it to the air that is fed through said burner, so that theoxygen combusts with said fuel in said fuel-rich zone, in an amount ofsaid oxygen which is less than 20% of the stoichiometric amount requiredfor complete combustion of said fuel, and adjusting the amount of airfed through said burner so that the stoichiometric ratio in said fuelrich zone is between 0.1 and 0.85, while maintaining or enlarging thesize of said fuel-rich zone compared to its size when combustion iscarried out in said combustion device without said oxygen feeding stepbut under otherwise identical conditions wherein the air flow from saidburner is predominantly axial and the oxygen is injected into said fuelat an angle of 45 to 135 degrees from the longitudinal axis of theburner.
 40. A method according to claim 39 wherein said angle is atleast 90 degrees from the longitudinal axis of the burner.
 41. A methodaccording to claim 39 wherein the stoichiometric ratio in said fuel richzone is between 0.4 and 0.75.